Viruses that hitchhike in mosquitoes to reach human hosts must ensure that they don't damage their insect vehicles while en route. For example, the mosquito-borne Sindbis virus maintains a latent but nonpathogenic infection in its mosquito carriers in order to stay circulating in the environment. Kevin M. Myles, Zach N. Adelman, and colleagues at Virginia Polytechnic Institute & State University now report that the Sindbis virus and its mosquito host work together to keep viral pathogenicity in check (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0803408105). First, the mosquito's immune system chops up the viral genome into small interfering RNAs. These viral siRNAs then interfere with the production of viral proteins that could hurt the mosquito vehicle, which results in greater distribution of the virus. When the researchers suppressed the production of viral siRNAs, there was more replication of the viral genome and an increased mortality in the insect host. "These results have broad implications for the control of mosquito-borne diseases," the authors note.